System and method for magnetic resonance i maging with prospective motion control

ABSTRACT

A system and method for reconstructing an image of a subject, in which motion has been prospectively corrected, using a magnetic resonance imaging (MRI] system are provided. An imaging pulse sequence is used to acquire image data in addition to navigator data using an ultra-fast navigator, such as a cloverleaf navigator. When motion is detected in the navigator data, a volume navigator (vNav] sequence is performed, from which motion estimates are generated and used to update the imaging pulse sequence to prospectively correct for the motion in subsequent repetition times.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on, claims priority to, and incorporatesherein by reference in its entirety, U.S. Provisional Application Ser.No. 61/981,452, filed Apr. 18, 2014, and entitled “SYSTEM AND METHOD FORMAGNETIC RESONANCE IMAGING WITH PROSPECTIVE MOTION CONTROL” and is basedon, claims priority to, and incorporates herein by reference in itsentirety, U.S. Provisional Application Ser. No. 62/005,165, filed May30, 2014, and entitled “SYSTEM AND METHOD FOR MAGNETIC RESONANCE IMAGINGWITH PROSPECTIVE MOTION CONTROL.”

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under HD074649 awardedby the National Institutes of Health. The government has certain rightsin the invention.

BACKGROUND

The present disclosure relates to systems and methods for magneticresonance imaging (“MRI”). More particularly, the disclosure relates tosystems and methods for tracking and controlling artifacts caused bymotion during a MRI procedure.

MRI uses the nuclear magnetic resonance (“NMR”) phenomenon to produceimages. When a substance such as human tissue is subjected to a uniformmagnetic field (“main magnetic field”), B₀, the individual magneticmoments of the nuclei in the tissue attempt to align with this magneticfield, but precess about it in random order at their characteristicLarmor frequency, ω. If the substance, or tissue, is subjected to anexcitation magnetic field, B₁, that is in the plane transverse to themain magnetic field, B₀, and that is near the Larmor frequency, ω, thenet aligned magnetic moment of the nuclei may be rotated, or “tipped,”into the transverse plane to produce a net transverse magnetic moment. Asignal is emitted by the excited nuclei, or “spins,” after theexcitation magnetic field, B₁, is terminated. The emitted signal may bereceived and processed to form an image.

When utilizing these emitted “MR” signals to produce images, magneticfield gradients (G_(x), G_(y), and G_(z)) are employed. Typically, theregion to be imaged is scanned by a sequence of measurement cycles inwhich these gradients vary according to the particular localizationmethod being used. The resulting set of received MR signals aredigitized and processed to reconstruct the image using one of many wellknown reconstruction techniques.

The measurement cycle used to acquire each MR signal is performed underthe direction of a pulse sequence produced by a pulse sequencer.Clinically available MRI systems store a library of such pulse sequencesthat can be prescribed to meet the needs of many different clinicalapplications. Research MRI systems include a library ofclinically-proven pulse sequences and they also enable the developmentof new pulse sequences.

Depending on the technique used, many MR scans currently require manyminutes to acquire the necessary data used to produce medical images.The reduction of this scan time is an important consideration, sincereduced scan time increases patient throughout, improves patientcomfort, and improves image quality by reducing motion artifacts. Manydifferent strategies have been developed to shorten the scan time.

For example, one popular category of pulse sequences are so-calledgradient echo sequences. Within this category, the spoiled gradient echoand three-dimensional spoiled gradient-recalled echo (SPGR) or fast lowangle shoot (FLASH) pulse sequences are often used in neuroimagingapplications. Specifically, the SPGR or FLASH sequence forms the basisof many 3D neuroimaging sequences, but acquisition times often stretchto several minutes. Lengthy acquisitions in neuroimaging applicationscan be particularly troublesome because it is imperative that thesubject remain motionless during the duration of the sequence. That is,in neuroimaging applications, motion can be particularly damaging to theresulting images because of the complexity of the structures beingimaged and studied in neuroimaging applications.

Motion-correction systems in MRI can be grouped into two generalmethods: prospective and retrospective. Retrospective methods useinformation about the subject's motion to estimate what k-space datawould have been measured if the subject had not moved during scanning.Prospective methods use motion-tracking data acquired during the scan tofollow the subject with the gradient axes of the sequence, measuring thedesired k-space data directly. Additionally, it is possible to combinethe two methods so that retrospective processing corrects residualerrors in the prospective system. A retrospective system can access allof the k-space data while performing reconstruction; a prospectivesystem must necessarily rely only on previous measurements to estimatethe current position of the patient. However, a prospective systemavoids the need to estimate missing k-space data, allowing for directreconstruction while avoiding possible sources of estimation error inthe k-space data.

Also, one can differentiate between two types of motion correctionproblems that arise in MRI: between-scan motion and within-scan motion.For between-scan motion, several retrospective motion correction methodsare available that register either slice-by-slice or volume-by-volume toestimate the data that would have been acquired in each volume if thesubject had not moved. Prospective motion correction can also beemployed for this problem, such as the orbital navigator system thatinserts 3-plane circular k-space navigators, or the PACE system thatregisters each completed EPI volume back to the first time-point and sorequires no navigators.

For in-scan motion, several methods are available, such as PROPELLER andthe like that use redundant sampling of the center of k-space duringeach repetition time (TR) and estimate motion-free k-space dataretrospectively. Also, prospective motion correction is available, suchas by using cloverleaf navigators. The use of cloverleaf navigators isuseful with SPGR/FLASH sequences.

However, in order to maximize SNR/time efficiency, short TR protocolsare often used in neuroimaging applications. Such short-TR protocols, bydefinition, have very-little dead time and, thus, force navigators to bevery short and provide limited k-space coverage. That is, as the TR isreduced, the effectiveness of the navigator is reduced because there isless information gathered by the navigator to use to form an estimate ofthe subject's head motion.

It would therefore be desirable to provide a system and method forcontrolling the competing constraints of neuroimaging applications thatdesire extended acquisition times and the need to control or compensateor correct for patient motion during such acquisitions.

SUMMARY

The present disclosure overcomes the aforementioned drawbacks byproviding a system and method for directing a magnetic resonance imaging(MRI) system to reconstruct an image of a subject, in which motion hasbeen prospectively corrected. The method includes acquiring image dataand navigator data from a subject by directing the MRI system to performan imaging pulse sequence that includes a navigator portion. Thenavigator data is processed to determine whether motion occurred whilethe image data was acquired. The imaging pulse sequence is updated whenprocessing the navigator data determines motion occurred. The imagingpulse sequence is updated by first acquiring volume navigator data bydirecting the MRI system to perform a volume navigator pulse sequencethat uses an echo-planar imaging (EPI) technique to acquire volumenavigator data from a volume-of-interest in the subject. The volumenavigator data is then processed to generate motion estimates of motionin the VOI. The imaging pulse sequence is then updated based on themotion estimates, wherein the updated imaging pulse sequenceprospectively corrects for the motion in the VOI using the motionestimates. The imaging pulse sequence, whether updated or not, is thenrepeated and navigator data processed to determine whether motionoccurred and whether the imaging pulse sequence should be furtherupdated until a desired amount of image data is acquired. An image ofthe subject is then reconstructed from the acquired image data.

In accordance with yet another aspect of the disclosure, a magneticresonance imaging (MRI) system is disclosed that includes a magnetsystem configured to generate a polarizing magnetic field about at leasta portion of a subject arranged in the MRI system, a magnetic gradientsystem including a plurality of magnetic gradient coils configured toapply at least one magnetic gradient field to the polarizing magneticfield, and radio frequency (RF) system configured to apply an RF fieldto the subject and to receive magnetic resonance signals from thesubject using a coil array. The system also includes a computer systemprogrammed to control the gradient system and the RF system to performan imaging pulse sequence that includes a navigator portion to acquireimage data and navigator data from a subject. The computer system isfurther programmed to process the navigator data to determine whetherthe subject moved while the image data was acquired. Upon determiningthat the subject moved, the computer system is configured to control thegradient system and the RF system to perform a volume navigator pulsesequence that uses an echo-planar imaging (EPI) technique to acquirevolume navigator data from a volume-of-interest in the subject, processthe volume navigator data to generate motion estimates of motion in theVOI, and update the imaging pulse sequence based on the motionestimates, wherein the updated imaging pulse sequence prospectivelycorrects for the motion in the VOI using the motion estimates. Thecomputer system is further configured to reconstruct an image of thesubject from the acquired image data.

The foregoing and other aspects and advantages of the invention willappear from the following description. In the description, reference ismade to the accompanying drawings which form a part hereof, and in whichthere is shown by way of illustration a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference is made therefore to the claimsand herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example of a magnetic resonance imaging(“MRI”) system.

FIG. 2 is an example of a three-dimensional spoiled gradient-recalledecho (“3D SPGR”) pulse sequence.

FIG. 3 is an illustrative example of an integrated pulse sequence thatincludes both imaging sequences, each played out during an imagingrepetition time (“TR”), and volumetric navigator sequences, each playedout during a volumetric navigator TR;

FIG. 4 is a flowchart setting forth the steps of an example method forperforming prospective motion correction using a pulse sequence such asthe one described with respect to FIG. 3; and

FIG. 5 is a flowchart setting forth the steps of an example method forperforming prospective motion correction based on an integrated pulsesequence that uses an ultra-fast navigator in an imaging pulse sequenceto detect motion and trigger the performance of one or more vNavsequences.

DETAILED DESCRIPTION OF THE INVENTION

Referring particularly now to FIG. 1, an example of a magnetic resonanceimaging (“MRI”) system 100 is illustrated. The MRI system 100 includesan operator workstation 102, which will typically include a display 104;one or more input devices 106, such as a keyboard and mouse; and aprocessor 108. The processor 108 may include a commercially availableprogrammable machine running a commercially available operating system.The operator workstation 102 provides the operator interface thatenables scan prescriptions to be entered into the MRI system 100. Ingeneral, the operator workstation 102 may be coupled to four servers: apulse sequence server 110; a data acquisition server 112; a dataprocessing server 114; and a data store server 116. The operatorworkstation 102 and each server 110, 112, 114, and 116 are connected tocommunicate with each other. For example, the servers 110, 112, 114, and116 may be connected via a communication system 140, which may includeany suitable network connection, whether wired, wireless, or acombination of both. As an example, the communication system 140 mayinclude both proprietary or dedicated networks, as well as opennetworks, such as the internet.

The pulse sequence server 110 functions in response to instructionsdownloaded from the operator workstation 102 to operate a gradientsystem 118 and a radiofrequency (“RF”) system 120. Gradient waveformsnecessary to perform the prescribed scan are produced and applied to thegradient system 118, which excites gradient coils in an assembly 122 toproduce the magnetic field gradients G_(x), G_(y), and G_(z) used forposition encoding magnetic resonance signals. The gradient coil assembly122 forms part of a magnet assembly 124 that includes a polarizingmagnet 126 and a whole-body RF coil 128.

RF waveforms are applied by the RF system 120 to the RF coil 128, or aseparate local coil (not shown in FIG. 1), in order to perform theprescribed magnetic resonance pulse sequence. Responsive magneticresonance signals detected by the RF coil 128, or a separate local coil(not shown in FIG. 1), are received by the RF system 120, where they areamplified, demodulated, filtered, and digitized under direction ofcommands produced by the pulse sequence server 110. The RF system 120includes an RF transmitter for producing a wide variety of RF pulsesused in MRI pulse sequences. The RF transmitter is responsive to thescan prescription and direction from the pulse sequence server 110 toproduce RF pulses of the desired frequency, phase, and pulse amplitudewaveform. The generated RF pulses may be applied to the whole-body RFcoil 128 or to one or more local coils or coil arrays (not shown in FIG.1).

The RF system 120 also includes one or more RF receiver channels. EachRF receiver channel includes an RF preamplifier that amplifies themagnetic resonance signal received by the coil 128 to which it isconnected, and a detector that detects and digitizes the I and Qquadrature components of the received magnetic resonance signal. Themagnitude of the received magnetic resonance signal may, therefore, bedetermined at any sampled point by the square root of the sum of thesquares of the I and Q components:

M=√{square root over (I ² +Q ²)}  (1);

and the phase of the received magnetic resonance signal may also bedetermined according to the following relationship:

$\begin{matrix}{\phi = {{\tan^{1}( \frac{Q}{I} )}.}} & (2)\end{matrix}$

The pulse sequence server 110 also optionally receives patient data froma physiological acquisition controller 130. By way of example, thephysiological acquisition controller 130 may receive signals from anumber of different sensors connected to the patient, such aselectrocardiograph (“ECG”) signals from electrodes, or respiratorysignals from a respiratory bellows or other respiratory monitoringdevice. Such signals are typically used by the pulse sequence server 110to synchronize, or “gate,” the performance of the scan with thesubject's heart beat or respiration.

The pulse sequence server 110 also connects to a scan room interfacecircuit 132 that receives signals from various sensors associated withthe condition of the patient and the magnet system. It is also throughthe scan room interface circuit 132 that a patient positioning system134 receives commands to move the patient to desired positions duringthe scan.

The digitized magnetic resonance signal samples produced by the RFsystem 120 are received by the data acquisition server 112. The dataacquisition server 112 operates in response to instructions downloadedfrom the operator workstation 102 to receive the real-time magneticresonance data and provide buffer storage, such that no data is lost bydata overrun. In some scans, the data acquisition server 112 does littlemore than pass the acquired magnetic resonance data to the dataprocessor server 114. However, in scans that require information derivedfrom acquired magnetic resonance data to control the further performanceof the scan, the data acquisition server 112 is programmed to producesuch information and convey it to the pulse sequence server 110. Forexample, during prescans, magnetic resonance data is acquired and usedto calibrate the pulse sequence performed by the pulse sequence server110. As another example, navigator signals may be acquired and used toadjust the operating parameters of the RF system 120 or the gradientsystem 118, or to control the view order in which k-space is sampled. Instill another example, the data acquisition server 112 may also beemployed to process magnetic resonance signals used to detect thearrival of a contrast agent in a magnetic resonance angiography (“MRA”)scan. By way of example, the data acquisition server 112 acquiresmagnetic resonance data and processes it in real-time to produceinformation that is used to control the scan.

The data processing server 114 receives magnetic resonance data from thedata acquisition server 112 and processes it in accordance withinstructions downloaded from the operator workstation 102. Suchprocessing may, for example, include one or more of the following:reconstructing two-dimensional or three-dimensional images by performinga Fourier transformation of raw k-space data; performing other imagereconstruction algorithms, such as iterative or backprojectionreconstruction algorithms; applying filters to raw k-space data or toreconstructed images; generating functional magnetic resonance images;calculating motion or flow images; and so on.

Images reconstructed by the data processing server 114 are conveyed backto the operator workstation 102 where they are stored. Real-time imagesare stored in a data base memory cache (not shown in FIG. 1), from whichthey may be output to operator display 112 or a display 136 that islocated near the magnet assembly 124 for use by attending physicians.Batch mode images or selected real time images are stored in a hostdatabase on disc storage 138. When such images have been reconstructedand transferred to storage, the data processing server 114 notifies thedata store server 116 on the operator workstation 102. The operatorworkstation 102 may be used by an operator to archive the images,produce films, or send the images via a network to other facilities.

The MRI system 100 may also include one or more networked workstations142. By way of example, a networked workstation 142 may include adisplay 144; one or more input devices 146, such as a keyboard andmouse; and a processor 148. The networked workstation 142 may be locatedwithin the same facility as the operator workstation 102, or in adifferent facility, such as a different healthcare institution orclinic.

The networked workstation 142, whether within the same facility or in adifferent facility as the operator workstation 102, may gain remoteaccess to the data processing server 114 or data store server 116 viathe communication system 140. Accordingly, multiple networkedworkstations 142 may have access to the data processing server 114 andthe data store server 116. In this manner, magnetic resonance data,reconstructed images, or other data may be exchanged between the dataprocessing server 114 or the data store server 116 and the networkedworkstations 142, such that the data or images may be remotely processedby a networked workstation 142. This data may be exchanged in anysuitable format, such as in accordance with the transmission controlprotocol (“TCP”), the internet protocol (“IP”), or other known orsuitable protocols.

As described, systems such as described above with respect to FIG. 1have been used to perform neuroimaging acquisitions with shortrepetition times (“TRs”), which force navigators to be very short and,therefore, provide limited k-space coverage. These navigators are oftenreferred to as “ultra-fast navigators.” The effectiveness of thenavigators at providing an estimate of the subject's head motion is,therefore, also limited.

In contrast to these ultra-fast navigators, echo-planar imaging(EPI)-based navigators, or volume navigators (“vNays”), have been usedto acquire a whole-head volume in roughly 275 ms and, thereby, allowhigh-accuracy motion tracking. However, vNays have previously only beenused in sequences with significant dead time provided by inflow times(“TI”) or TR gaps in which the entire vNav could be inserted. As will bedescribed, however, the present disclosure provides a way to insert avNav into a 3D FLASH/SPGR sequence with only marginal impact on SNR ortime. By doing so, the present disclosure provides a system and methodfor prospective motion correction of in-scan motion using a 3DFLASH/SPGR pulse sequence with vNays that does not substantially extendacquisition times.

An example of a pulse sequence employed to direct an MRI system toacquire image data in accordance with some configurations of the presentinvention is illustrated in FIG. 2. The pulse sequence includes an RFexcitation pulse 202 that is played out in the presence of aslab-selective gradient 204 in order to produce transverse magnetizationin a volume-of-interest. The slab-selective gradient 204 includes arephasing lobe 206 that acts to rephase unwanted phase dispersionsintroduced by the slab-selective gradient 204 such that signal lossesresultant from these phase dispersions are mitigated.

Following excitation of the nuclear spins in the volume-of-interest, aphase-encoding gradient 208 is applied to spatially encode a nuclearmagnetic resonance echo signal 210 at a given echo time (TE)representative of a gradient-recalled echo along one direction in thevolume-of-interest. At the same time, a partition-encoding gradient 212is applied to spatially encode the echo signal 210 along a second,orthogonal direction in the volume-of-interest. By way of example, thephase-encoding gradient 208 may spatially encode the echo signal 210along the y-direction, while the partition-encoding gradient 212 mayspatially encode the echo signal 210 along the z-direction. A readoutgradient 214 is also applied after a dephasing gradient lobe 216 tospatially encode the echo signal 210 along a third, orthogonal directionin the volume-of-interest. By way of example, the readout gradient 214may spatially encode the echo signal along the x-direction. The echosignal 210 is sampled during a data acquisition window.

Spoiler gradients 218, 220, 222 may be played out along thepartition-encoding, phase-encoding, and frequency-encoding directions todephase any residual transverse magnetization in the volume-of-interestto prevent signal contamination from one repetition time (TR) period tothe next.

Referring now to FIG. 3, the pulse sequence descried above with respectto FIG. 2 can, in accordance with the present disclosure, be combinedwith vNav sequences to yield a pulse sequence for performing prospectivemotion correction in SPGR/FLASH without extended TI or TR gaps.Specifically, FIG. 3 provides a timing diagram of an integrated vNav anda SPGR/FLASH sequence 300, showing common pulses and TRs, but variedgradients, for interleaved imaging and vNav sequences. In particular, aplurality of imaging sequences 302, such as described above with respectto FIG. 2 are illustrated. Integrated therewith are a plurality of vNavsequences 304. As will be described, the scan time of the imagingsequence 302 is selected such that an imaging TR 306 allows vNavsequences 304, each having a vNav TR 308, to be inserted at any desiredpoint. Furthermore, the RF pulses used in the vNav sequences 304 and theimaging sequences 302 may be the same. To this end, the steady-state ofthe imaging sequences 302 is maintained, even when vNav sequences 304are inserted in the integrated sequence 300.

As an example, the vNav sequence 304 may be a 3D-encoded echo-planarimaging (EPI) pulse sequence with a 32³ matrix and may be acquired with¾ partial Fourier encoding in the partition direction. As such, in thisexample, the vNav sequence 304 may have 25 three-dimensional excitationpulses. In this example, the vNav TR 308 is assumed to be 11 ms and theTR 306 for each imaging sequence 302 is assumed to match the vNav TR308. If the pulses of both sequences 302, 304 are matched, a train of 25vNav TRs (i.e., one vNav) can be played instead of a TR 306 of animaging sequence 302 without disturbing the steady state of the imagingsequence 302. Expanding on this example, as long as the TR 306 of theimaging sequence 302 is 11 ms or longer, matching the vNav TR 308 to theimaging TR 306 can be readily achieved by adding TRs 310 to the vNavsequence 304 after its readouts and not simply attempting to fit thevNav TR 308 within dead time. Again, as long as the scan time of theimaging sequence 302 meets the minimum TR requirement, vNav TRs 308 canbe inserted at any desired point while maintaining the steady-state ofthe imaging sequence.

Inserting the vNav sequences 304 as separate TRs 308 at any time allowsa great deal of flexibility, but also increases overall scan time. Inprevious acquisitions using vNav sequences, the navigators were insertedin TR or TI gaps and, thus, did not increase scan time. However,continuing with the non-limiting example provided above, 25 additionalTRs are added to the total scan time with every vNav sequence while notgaining any additional imaging signal. To allow grater flexibility, aswill be described, the user may set how many imaging TRs will be playedbetween each vNav sequence, and the overall scan time is updated toinform users of the trade-off between tracking accuracy and scan time.

In some embodiments, however, an additional time savings can be realizedby selectively performing a limited number of vNav sequences 304 suchthat the vNav sequences 304 are performed only when a predeterminedamount of motion is occurring. In these instances, motion detection canbe provided by inserting an ultra-fast navigator, such as a cloverleafnavigator, into each imaging sequence 302. Examples of cloverleafnavigators are described in U.S. Pat. Nos. 6,771,068 and 6,958,605, bothof which are herein incorporated by reference in their entirety. Theultrafast-navigator can be used as a high-sensitivity motion detectorthat can be relied on to determine when motion occurs without asignificant scan time cost. When motion is detected, one or more vNavsequences 304 can be performed to provide a more accurate measurement ofthe motion, as described above in detail. In this manner, the more timeconsuming vNav sequences 304 can be performed only when a predeterminedamount of motion is detected by an ultra-fast navigator, therebyreducing overall scan time by eliminating the performance of unnecessaryvNav sequences 304. Integrating an ultra-fast navigator into the imagingsequences 302 also has the added benefit of enabling resonance frequencycorrection.

Specifically, referring to FIG. 4, a flowchart is provided that setsforth an example of a process for performing prospective motioncorrection using the pulse sequence described with respect to FIG. 3.The prospective motion-corrected process 400 begins with user selectionof the imaging pulse sequences and parameters for the sequence atprocess block 402. As described above, the imaging pulse sequence may bea FLASH or SPGR or other name for such gradient-echo sequences. Once theimaging sequence and parameters are selected, at process block 404, theuser is prompted to select how many imaging TRs will be played betweeneach vNav TR. Using this information, at process block 406, the overallscan time is updated to inform users of the trade-off between trackingaccuracy and scan time. As indicated by decision block 408, the user mayadjust the number and spacing of the vNav sequences to reach a desiredoverall scan time until the user has developed an overall integratedpulse sequence that is desired to be run.

At process block 410, scanning begins using the overall integrated pulsesequence built by the user as described above. Using the data acquiredfrom each performance of the vNav pulse sequence, motion estimates aregenerated at process block 412. Specifically, the motion estimates aresent back to the scanner as they are generated on the imagereconstruction computer, and are immediately applied at process block414 to the coordinates of the next imaging sequence to provide real-timeand prospective correction of subject motion that is identified. Thisprocess is repeated until, at decision block 416, all data has beenacquired. This method is applicable both to three-dimensionalacquisitions and to simultaneous multi-slice acquisitions, includingwhen such acquisitions include in-place acceleration.

To test the efficacy of the above-described process, a human volunteerwas scanned in a 3T TIM Trio (Siemens Healthcare, Erlangen, Germany)using a 32-channel head matrix. The imaging sequence was a FLASH pulsesequence that used a 15 degree flip angle, 11 ms TR, 3.43 ms TE, 200Hz/px bandwidth, 256 mm×256 mm×176 mm FOV, and 1 mm isotropicresolution, and 2×GRAPPA acceleration for a total scan time of 4:48. Onevolume was acquired with this protocol while the subject remained still.Thereafter, a vNav sequence was inserted after every 136 FLASH TRs(approximately 1.5 seconds), increasing the scan time to 5:38, andacquired another volume while the subject remained still. Then, two morevolumes were acquired with the vNays sequences inserted, during both ofwhich the subject was prompted to change their head position everyminute, repeating the same motion pattern in both scans. In the first ofthese with-motion scans, correction was applied, using the processdescribed above, for the subject's motion in real time, while in thesecond with-motion scan not apply the update.

Referring now to FIG. 5, a flowchart is illustrated as setting forth thesteps of an example method for performing prospective motion correctionbased on an integrated pulse sequence that uses an ultra-fast navigatorin an imaging pulse sequence to detect motion and trigger theperformance of one or more vNav sequences. The method begins with userselection of the imaging pulse sequences and parameters for the sequenceat process block 502. As described above, the imaging pulse sequence maybe a FLASH, SPGR, or other such gradient-echo pulse sequence. Selectingthe imaging pulse sequences also includes selecting an ultra-fastnavigator to be included in each imaging pulse sequence. For example,the ultra-fast navigator may include a cloverleaf navigator.

At process block 504, scanning begins using the selected imaging pulsesequences. Motion of the subject is monitored using the data acquiredfrom the ultra-fast navigators acquired in each performance of theimaging pulse sequence, as indicated at step 506. A determination ismade at decision block 508 whether a predetermined amount of motion hasoccurred and been detected in the acquired navigator data. If not,imaging continues at step 504 and motion is monitored again at step 506.

If, however, a predetermined amount of motion is detected, then one ormore vNav sequences are performed, as indicated at step 510, asdescribed above. Using data acquired from the vNav pulse sequences,motion estimates are generated at process block 512. Specifically, themotion estimates are sent back to the scanner as they are generated onthe image reconstruction computer, and are immediately applied atprocess block 514 to the coordinates of the next imaging sequence toprovide real-time and prospective correction of subject motion that isidentified. This process is repeated until, at decision block 516, alldata has been acquired. This method is applicable for boththree-dimensional acquisitions and simultaneous multi-sliceacquisitions, including when such acquisitions include in-placeacceleration.

Thus, a system and method is provided for performing prospective motioncorrection in FLASH, SPGR, or FLASH/SPGR-like pulse sequences. Unlikeprevious navigator methods that attempted to insert ultra-fastnavigators into the imaging (FLASH/SPGR) TR, the present disclosureprovides a way to insert multiple matched vNav TRs into the overallpulse sequence train and, thus, preserves the steady state. The presentdisclosure recognizes that a navigator of substantial length, includingvNays, can be inserted or interleaved in a dense sequence imagingsequence (like FLASH), by substituting FLASH TRs for vNav TRs, whilepreserving the magnetization steady state. For example, to preserve themagnetization steady state, it is advantageous to match the vNav andFLASH TR times. Also, it is advantageous to ensure that the waveform ofthe excitation pulses match between the vNav and FLASH TRs. Further, ifany gradients are used during the excitation pulse, it is advantageousto match these gradients. In addition, any RF spoiling between the vNavand FLASH TRs may be advantageously synchronized.

Unlike in previous applications of vNays, where there was no scan-timeincrease, the additional TRs to accommodate the vNav sequence in theintegrated FLASH-vNav pulse sequence does increase overall scan time.However, this drawback is overcome by the high registration accuracyenabled by whole-head navigators readily achieved with the vNavsequence. Furthermore, the additional scan time is still significantlyless than what would be required for an MR technologist to requestpatient compliance and rescan. Additionally, the provided, integratedvNav-FLASH/SPGR sequence allows users the flexibility to trade-off scantime for tracking accuracy based on the needs of their application andthe subject population involved.

In some configurations, the additional scan time introduced by the vNavsequences is reduced by incorporating an ultra-fast navigator, such as acloverleaf navigator, into each imaging pulse sequence. The navigatordata is then relied upon to monitor motion, such that vNav sequences areperformed only when a significant amount of subject motion is detected.In this manner, the benefits of the vNav sequences can be realizedwithout adding unnecessary amounts of additional scan time.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

1. A method for reconstructing an image of a subject, in which motionhas been prospectively corrected, using a magnetic resonance imaging(MRI) system, the steps of the method comprising: (a) acquiring imagedata and navigator data from a subject by directing the MRI system toperform an imaging pulse sequence that includes a navigator portion; (b)processing the navigator data to determine whether motion occurred whilethe image data was acquired; (c) upon determining motion occurred instep (b), updating the imaging pulse sequence by: (i) directing the MRIsystem to perform a volume navigator pulse sequence that uses anecho-planar imaging (EPI) technique to acquire volume navigator datafrom a volume-of-interest in the subject; (ii) processing the volumenavigator data to generate motion estimates of motion in the VOI; (iii)updating the imaging pulse sequence based on the motion estimates,wherein the updated imaging pulse sequence prospectively corrects forthe motion in the VOI using the motion estimates; (d) repeating steps(a) through (c) until a desired amount of image data is acquired; and(e) reconstructing an image of the subject from the acquired image data.2. The method as recited in claim 1, wherein the imaging pulse sequenceincludes a gradient echo pulse sequence.
 3. The method as recited inclaim 1, wherein the navigator portion of the imaging pulse sequenceincludes performing a cloverleaf navigator.
 4. The method as recited inclaim 1, wherein the image data is acquired during an imaging repetitiontime (TR) and the volume navigator data is acquired during a volumenavigator TR that is coordinated with the imaging TR to preserve asteady-state of magnetization in the VOI associated with the imagingpulse sequence.
 5. The method as recited in claim 4, wherein the volumenavigator TR is selected to match the imaging TR.
 6. The method asrecited in claim 1, wherein the VOI includes a head of the subject, andthe volume navigator pulse sequence is configured to acquire the volumenavigator data from the head.
 7. A magnetic resonance imaging (MRI)system, comprising: a magnet system configured to generate a polarizingmagnetic field about at least a portion of a subject arranged in the MRIsystem; a magnetic gradient system including a plurality of magneticgradient coils configured to apply at least one magnetic gradient fieldto the polarizing magnetic field; a radio frequency (RF) systemconfigured to apply an RF field to the subject and to receive magneticresonance signals from the subject using a coil array; a computer systemprogrammed to: control the gradient system and the RF system to performan imaging pulse sequence that includes a navigator portion to acquireimage data and navigator data from a subject; process the navigator datato determine whether the subject moved while the image data wasacquired; upon determining that the subject moved, control the gradientsystem and the RF system to: perform a volume navigator pulse sequencethat uses an echo-planar imaging (EPI) technique to acquire volumenavigator data from a volume-of-interest in the subject; process thevolume navigator data to generate motion estimates of motion in the VOI;update the imaging pulse sequence based on the motion estimates, whereinthe updated imaging pulse sequence prospectively corrects for the motionin the VOI using the motion estimates; and reconstruct an image of thesubject from the acquired image data.
 8. The system as recited in claim7, wherein the imaging pulse sequence includes a gradient echo pulsesequence.
 9. The system as recited in claim 7, wherein the navigatorportion of the imaging pulse sequence includes performing a cloverleafnavigator.
 10. The system as recited in claim 7, wherein the image datais acquired during an imaging repetition time (TR) and the volumenavigator data is acquired during a volume navigator TR that iscoordinated with the imaging TR to preserve a steady-state ofmagnetization in the VOI associated with the imaging pulse sequence. 11.The method as recited in claim 10, wherein the volume navigator TR isselected to match the imaging TR.
 12. The method as recited in claim 7,wherein the VOI includes a head of the subject, and the volume navigatorpulse sequence is configured to acquire the volume navigator data fromthe head.